从WeakReference Java获取参考

Question

Is it same ways to use weekreference?

first way: in the contstructor create WeakReference obj and immediately get my Callbacks obj.

private static final class LoadImageFromUrl extends AsyncTask<Void, Void, Bitmap> {

    private Callbacks mCallbacks;

    public LoadImageFromUrl(String url, Callbacks callbacks) {
        mCallbacks = new WeakReference<>(callbacks).get();
    }

    @Override
    protected void onPostExecute(Bitmap bitmap) {
        mCallbacks.successfully(bitmap);
    }

second way: in the contstructor create WeakReference obj and get my Callbacks obj when i need something do with it.

private static final class LoadImageFromUrl extends AsyncTask<Void, Void, Bitmap> {

    private WeakReference<Callbacks> mCallbacks;

    public LoadImageFromUrl(String url, Callbacks callbacks) {
        mCallbacks = new WeakReference<>(callbacks);
    }

    @Override
    protected void onPostExecute(Bitmap bitmap) {
        mCallbacks.get().successfully(bitmap);
    }

I think it similar ways, but not exactly.

Answer 1

In the first case, it is useless to make a WeakReference because you get the value into it immediately, to store it into a strong reference (the field of your class)

In the second case, it is a real weak reference : if there is no other strong reference of your callback, it could be garbage collected. So mCallbacks.get() could return null in this case.

Answer 2

Why do you want to keep your callback s as WeakReference ? Have you read anything about Strong / Weak / Soft / PhantomReferences ?

I would advice you to get some knowledge about it and then rethinking your solution. Example article

Answer 3

It's the second version with the addition of a check whether the callback still exists.

private static final class LoadImageFromUrl extends AsyncTask<Void, Void, Bitmap> {

    private WeakReference<Callbacks> mCallbacks;

    public LoadImageFromUrl(String url, Callbacks callbacks) {
        mCallbacks = new WeakReference<>(callbacks);
    }

    @Override
    protected void onPostExecute(Bitmap bitmap) {
        Callbacks cb = mCallbacks.get();
        if (cb != null) cb.successfully(bitmap);
    }
}

But that is a legitimate yet dangerous implementation because the caller needs to keep a strong reference. And that not always the case especially with callbacks. For example

void onCreate(..) {
    ... 
    new LoadImageFromUrl(new Callbacks() { void foo() .... }).execute();
    ...
}

will fail because nothing keeps a strong reference to the anonymous inner implementation. And that's a common way to implement callbacks.

Semi related: Make sure not to make sure not to extend the LoadImageFromUrl task as inner class because that will cause it to leak the containing class due to the implicit ContainingClass.this reference in every inner class. That would defeat the purpose of weak references.